1. Field of the Invention
The present invention relates to a charge/discharge control circuit that is capable of controlling charging and discharging of a secondary battery, and a charge/discharge type power supply using the circuit.
2. Description of the Related Art
As a conventional charge/discharge power supply, there has been known a charge/discharge type power supply 200 as shown in FIG. 2. In the charge/discharge type power supply 200, the positive pole of a secondary battery 204 is connected with an external connection positive terminal 205 to which a charger 208 or a load 207 is connected, and the negative pole of the secondary battery 204 is connected with an external connection negative terminal 206 to which the charger 208 or the load 207 is connected through a MOS transistor 202A and its parasitic diode 203A as well as a MOS transistor 202B and its parasitic diode 203B. In addition, a charge/discharge control circuit 201 is connected in parallel with the secondary battery 204. The charge/discharge control circuit 201 can detect a voltage across the secondary battery 204 and voltages across the MOS transistors 202A and 202B.
When the secondary battery 204 is in an over-discharge state, in which the battery output is lower than a predetermined voltage value (hereinafter referred to as xe2x80x9cover-discharge statexe2x80x9d), and/or when the secondary battery 204 is in an over-current state, in which the voltages across the MOS transistors 202A and 202B are higher than a predetermined voltage value (hereinafter referred to as xe2x80x9cover-current statexe2x80x9d), a signal is outputted from the discharge control terminal 213A of the charge/discharge control circuit 201 so that the MOS transistor 202A is turned off.
When the secondary battery 204 is in an over-charge state, in which the battery output is higher than the predetermined voltage value (hereinafter referred to as xe2x80x9cover-charge statexe2x80x9d), a signal is outputted from the charge control terminal 213B of the charge/discharge control circuit 201 so that the MOS transistor 202B is turned off.
In FIG. 2, an n-channel type is used for the MOS transistors. In another example, two p-channel MOS transistors each having a parasitic diode are connected in series between the positive pole of the secondary battery 204 and the external connection positive terminal 205 to which the charger 208 or the load 207 is connected.
However, the conventional charge/discharge power supply thus structured requires a diode that provides a discharge path when a charge control switch element is off, and a diode that provides a charge path when a discharge control switch element is off.
Accordingly, in the case of discharging the secondary battery which is in the over-charge state, since the charge control MOS transistor is off, a current flows through the parasitic diode connected in parallel with the charge control MOS transistor, with the result that the discharge efficiency is impaired due to heating at the parasitic diode. Also, the discharge current is restricted to the maximum permissible current value of the parasitic diode.
Also, in the case of charging the secondary battery which is in the over-discharge state, since the discharge control MOS transistor is off, a charge current flows through the parasitic diode connected in parallel with the discharge control MOS transistor, with the result that the charge efficiency is impaired due to heating at the parasitic diode. Also, the charge current is restricted to the maximum permissible current value of the parasitic diode.
In addition, in the conventional charge/discharge power supply, because the charge operation and the discharge operation are on/off controlled by different switching elements, two signal lines consisting of a charge control signal line and a discharge control signal line are required. For that reason, there are required two output terminals of the charge/discharge control circuit, two wirings extending from the charge/discharge control circuit to the switching elements, and two switching elements. Also, the possibility is so much the higher that the wiring functions as an antenna and high-frequency noises (for example, noises which occur when suffering electric waves of a cellar phone are received) come into the charge/discharge control circuit, to thereby lead to the malfunction of the on/off control of the switch elements.
Under the above circumstances, the present invention has been made to solve the above problems with the conventional power supply, and therefore an object of the present invention is to provide a charge/discharge type power supply that eliminates charge operation and discharge operation through diodes and controls both the charge operation and the discharge operation and the discharge operation of a secondary battery by one signal line.
In order to solve the above problem, according to the present invention, there is provided a charge/discharge type power supply that includes a MOS transistor (hereinafter referred to as xe2x80x9cfour-terminal MOS transistorxe2x80x9d) of which a source, a drain, a gate and a body are separated from each other as a switching element that controls both of the charge operation and the discharge operation, and provides a charge/discharge control circuit with a function of controlling the charge/discharge operation in accordance with a voltage of a secondary battery, a function of detecting a voltage that varies in accordance with a current that flows in the switching elements to control the charge/discharge operation, and a function of recognizing whether a charger or a load is connected to an external connection terminal to control the charge/discharge operation; in which one signal line is used for controlling the on/off operation of the switching element, and both of the charge operation and the discharge operation can be controlled without any diode.
In the charge/discharge type power supply thus structured, since the charge operation and the discharge operation through a diode are eliminated, the charging efficiency is enhanced to terminate the charge operation in a short period of time, and the discharging efficiency is enhanced to elongate the lifetime of the secondary battery. Also, since both of the charge operation and the discharge operation are controlled by using one signal line, the number of output terminals of the charge/discharge control circuit, the number of wirings extending from the charge/discharge control circuit to the switching elements and the number of switching elements are reduced, a chip area, a PKG area and a mounted area of the charge/discharge control circuit are reduced, and the number of parts to be used is reduced, thereby making it possible to downsize the charge/discharge type power supply and reduce the costs. Also, since the number of wirings is reduced, the rate of malfunction occurrence due to a high frequency noise is lessened.